This invention relates to a water heating system and, more specifically, to a water heating system that operates over a broad modulation range with excellent stability, reliability, and cost-efficiency.
Hot water temperature control devices have conventionally included heat exchangers to accomplish heat transfer between water which rapidly flows within tubes and a heat source, either steam or gas, exposed to the outside of the tubes. These systems, generally termed "instantaneous", produce fluctuating temperatures as a result of fluctuating flow and input energy. For example, if the system has an increased change in flow (increase demand for hot water), the temperature of the water will start to decay immediately since the temperature droop is a function of the rate of change of load (flow). In fact, if the load changed instantaneously from 0 to 100% (or to maximum) the outlet water temperature could momentarily drop to close to the inlet water temperature.
Because of the delay (time to increase energy as a result of increased flow and time for water to absorb energy), there is a limit to the gain (amount of energy input per unit of temperature change), which causes droop in the system. For instance, if a device is set for 140.degree. temperature output at low flow, there typically could be a 20.degree.-25.degree. droop under steady state conditions, meaning for a 100% flow there would be a drop in the output temperature of 20.degree.-25.degree.. The temperature errors resulting from poor dynamic response are superimposed on the steady state temperature error that results from the low gains necessary for system stability.
As a result of such poor temperature control, storage tanks are usually employed for use with the instantaneous system to store heated water at a fixed temperature; in one embodiment water is pumped at a constant rate through the system to keep the temperature constant. Other methods include heating the stored water without pumping means and relying on natural convection to accomplish temperature control. Because the use of the storage tank does not by itself solve the problem of temperature control, devices, such as described in U.S. Pat. No. 4,305,547 (the "'547 patent") have been established to improve temperature control. In the '547 patent, the inventor provided an improvement over thermostat and plumbing control devices, a system wherein a combined set point and feed forward control is established that minimizes fluctuations in the temperature of the hot water by anticipating changes in BTU requirements. Such a system is based on an indirect (liquid or steam) method of supplying the energy source to the heat exchanger. In contrast, the tenuous nature of the energy input in a direct fired format such as utilized herein makes temperature control significantly more difficult and requires an even greater degree of sophistication than that described in the '547 patent.
Another problem of prior art systems, whether condensing or noncondensing, relates to total system efficiency, i.e. unit efficiency and distribution system efficiency. These efficiencies affect significantly the cost of fuel per delivered gallon of water. Typically, efficiencies are based upon laboratory conditions at rated (or maximum) load--a continuous operation of rated load. However, in the commercial application for potable water, the load diversity (meaning the load profile) is anything but continuous or constant, i.e., it fluctuates greatly over a period of time. For instance, the loads are higher in the mornings because of concentrated water use whereas in the afternoon the loads are lower since less people require water. Because all systems supply only the energy used, the heating (the input energy) must cycle on and off to supply the reduced load in the afternoon or, as the case may be, the increased load in the mornings. Normally, as load decreases, the unit (heat) cycles on and off to meet load; total energy supplied is sought to equal the reduced energy utilized. It is understood in the art that such cycling reduces efficiency.
Also, as a result of the characteristics of some prior art devices, particularly non-condensing systems, aside from the drawbacks of utilizing a storage tank and distribution and recirculation pumping, system efficiency is inadequate. Poor temperature characteristics and general unawareness of the instantaneous temperature in the distribution systems requires that the temperature be maintained significantly higher than necessary to prevent decay to unacceptable levels of temperature under load. The difference between this distribution temperature and the required use temperature produces continuous energy losses throughout the distribution system. These losses and increased probabilities of scalding are a consequence of existing technology.
Other problems of present devices relate to efficiency performance. For instance, the energy not absorbed by the fluid and not extracted by the flue are lost to the ambient air because the gases are in heat exchange relation not only with the fluid but also the ambient air. In addition, most gas-fired systems attempt to increase the surface area of the gas side of the tubes (to increase the ability of the gas to transfer its heat) by using fins, which have the characteristic of trapping the flue products causing carbon buildup. The greater the build-up of carbon, the worse the heat transfer becomes. As a result, there is a loss of efficiency and users are left with the laborious task of opening and cleaning the heat exchanger.
These problems have been addressed previously, in U.S. Pat. No. 4,852,524 (the "'524 patent"), also assigned to the present assignee Aerco International, Inc. While the water heating system disclosed in the '524 patent was a substantial improvement over the prior art, the present invention seeks to go even further and provides a water heating system with even greater stability, reliability, and cost-efficiency than the one disclosed in the '524 patent.